PROJECT SUMMARY Antibody?drug conjugates (ADCs) are monoclonal antibodies (mAbs) that are covalently linked to cell-killing drugs and have emerged as a major modality in anti-cancer treatment. This approach combines high specificity of mAbs against their antigen targets with highly potent cytotoxic drugs, resulting in ?armed? mAbs that deliver the payload (drug) to tumor cells with enriched levels of the antibody target. As antibody engineering and linker-payload optimization are becoming mature, the discovery and development of new ADCs is increasingly dependent on the identification and validation of new targets that are suitable to this approach. LGR4 (leucine-rich repeat containing, G protein-coupled receptor 4) is a seven transmembrane domain receptor that is highly upregulated expression in the majority of solid tumors, including colorectal, lung, and ovarian cancers. LGR4 functions as a receptor of the R-spondin group of stem cell factors to potentiate Wnt signaling. Remarkably, LGR4 is rapidly internalized into intracellular vesicles in a constitutive fashion. The highly upregulated expression of LGR4 in tumor cells and its robust internalization make it a potential target of the ADC approach for the treatment of major types of solid tumors. We have generated and characterized a panel of highly potent and specific mAbs against native LGR4. Preliminary data showed that LGR4 mAbs conjugated with a potent cytotoxin were able to inhibit the growth of several cancer cell lines with high LGR4 expression in vitro and tumor xenografts in vivo. Here we propose to determine the potency, efficacy, and therapeutic window of anti-LGR4 ADCs in xenograft models of patient-derived tumors to establish proof-of-principle for the use of LGR4-targeded ADCs for the treatment of LGR4-high tumors. These results and conclusions may, for the first time, validate LGR4 as a novel target for the development of ADC-based therapeutics that has the potential to treat a large population of cancer patients.